Slip-modified, electrically conductive polyoxymethylene

ABSTRACT

Polyoxymethylene with addition of conductivity black and of a lubricant mixture composed of a lubricant with predominantly external lubricant action and of a lubricant with predominantly internal action, and also with addition of an impact-modifier component provides a high level of electrical conductivity with good mechanical and tribological qualities. It is used for functional components with requirements for good electrical conductivity with good wear performance.

The invention relates to an electrically conductive polyoxymethylenemolding composition whose electrical conductivity is maintained viaaddition of a lubricant mixture composed of a lubricant withpredominantly external lubricant action and of a lubricant withpredominantly internal lubricant action in order to improve abrasionperformance, and to its use.

Traditional materials, such as metals, are increasingly undergoingsuccessful replacement by plastics. Because the electrical resistanceswithin plastics are usually very high, there is a risk of electrostaticcharging, and this can be disruptive in certain application sectors, orcan even be dangerous. Attempts are therefore being made to improve theelectrical conductivity of plastics. A suitable method of reducinginternal electrical resistance is the addition of metal powders andmetal fibers, carbon fibers, graphite, or carbon black. Thelast-mentioned is in particular capable of universal use and processingto provide polymers with conductivity. The use of highly structuredcarbon blacks means that the amounts required are markedly lower thanfor graphite. A disadvantage with the use of highly structured carbonblacks is their sensitivity to processing effects. On the one hand, thedispersion of the carbon black has to be sufficiently good, and on theother hand excessive shear must not be allowed to break down the carbonblack agglomerates. For this reason, use is often made of lubricantadditives intended to contribute to low shear within the melt.

Many of the moldings produced from electrically conductivepolyoxymethylene are also subject to tribological stresses. It is wellknown that additions of lubricants can improve the tribological, i.e.sliding and friction, properties of thermoplastics. However, the problemis to find the correct selection and combination of lubricants for theparticular plastic. The processing aids used in POM are not suitable forreducing abrasion on moldings subject to wear. Only a few lubricants arecapable of reducing abrasion when used with conductivity blackincorporated into POM. It is likely that they disrupt the bondingbetween matrix and carbon black. This may well also be the reason forthe reduction in mechanical properties. Careful use of the lubricantsand optimized selection of the same are therefore required.

Another problem with the conductivity blacks is the fall-off intoughness. This can be compensated by addition of elastomers.

U.S. Pat. No. 4,828,755 describes a mixture where polyethylene glycoland non-polar polyethylene wax are proposed for incorporation ofconductivity black into polyoxymethylene. As is shown by the lowelectrical resistance values, the lubricants achieve incorporation ofthe carbon black into the matrix under gentle conditions, but abrasionfrom the resultant moldings is high, and mechanical properties, and alsoheat resistance, are severely reduced over the base material.

The object of the present invention was to improve abrasion performanceand reduce the fall-off in mechanical properties while retaining thegood electrical conductivity of a polyoxymethylene modified withconductivity black.

The object of the invention was achieved by using a lubricant mixturecomposed of a lubricant with predominantly external lubricant action,i.e. surface-active lubricants, and a lubricant with predominantlyinternal lubricant action, i.e. viscosity-reducing lubricants, that isto say lubricants whose lubricant effect acts predominantly within themelt.

The invention provides a molding composition composed of 30 to 89 partsby weight of a polyoxymethylene (A) and from 2 to 10 parts by weight,preferably from 3 to 5 parts by weight, of a conductivity black (B), andalso from 0.5 to 6 parts by weight, preferably from 3 to 5 parts byweight, of a lubricant mixture (C) composed of a lubricant withpredominantly internal lubricant action and of a lubricant withpredominantly external lubricant action, and from 1 to 12 parts byweight, preferably from 5 to 10 parts by weight, of an impact-modifiercomponent (D). The mixing ratio of lubricant with predominantly internallubricant action to lubricant with predominantly external lubricantaction may be from 0:100 to 100:0 parts by weight. The moldingcomposition may comprise additives and processing aids (E) in amounts offrom 0.005 to 50 parts by weight. Components (A) to (E) here always givea total of 100 parts by weight.

As stated at the outset, polyoxymethylene is a suitable component (A).

The polyoxymethylenes (POMs), for example as described in DE-A 29 47490, are generally unbranched linear polymers, generally containing atleast 80%, preferably at least 90%, of oxymethylene units (—CH₂O—). Theterm polyoxymethylenes here encompasses homopolymers of formaldehyde orof its cyclic oligomers, such as trioxane or tetroxane, and alsocorresponding copolymers.

Homopolymers of formaldehyde or of trioxane are polymers whose hydroxyend groups have been stabilized chemically in a known manner to preventdegradation, e.g. by esterification or etherification. Copolymers arepolymers of formaldehyde or of its cyclic oligomers, in particulartrioxane, with cyclic ethers, with cyclic acetals, and/or with linearpolyacetals.

These POM homo- or copolymers are known per se to the skilled worker andare described in the literature. Very generally, these polymers have atleast 50 mol % of —CH₂O— repeat units in the main polymer chain. Thehomopolymers are generally prepared by polymerization of formaldehyde ortrioxane, preferably in the presence of suitable catalysts.

For the purposes of the invention, POM copolymers are preferred ascomponent (A), in particular those which, besides the —CH₂O— repeatunits, also contain up to 50 mol %, preferably from 0.1 to 20 mol %, andin particular from 0.5 to 10 mol %, of

repeat units, where R¹ to R⁴, independently of one another, are ahydrogen atom, a C₁-C₄-alkyl group, or a halo-substituted alkyl grouphaving from 1 to 4 carbon atoms, and R⁵ is —CH₂—, —CH₂O—, aC₁-C₄-alkyl-substituted or C₁-C₄-haloalkyl-substituted methylene group,or a corresponding oxymethylene group, and n has a value in the rangefrom 0 to 3. These groups may advantageously be introduced into thecopolymers via ring-opening of cyclic ethers. Preferred cyclic ethersare those of the formula

where R¹ to R⁵ and n are as defined above. Merely by way of example,mention may be made of ethylene oxide, propylene 1,2-oxide, butylene1,2-oxide, butylene 1,3-oxide, 1,3-dioxane, 1,3-dioxolane, and1,3-dioxepan as cyclic ethers, and also linear oligo- or polyformals,such as polydioxolane or polydioxepan as comonomers.

Particularly advantageous copolymers are those of from 99.5 to 95 mol %of trioxane and from 0.5 to 5 mol % of one of the above-mentionedcomonomers.

Other suitable components (A) are oxymethyleneterpolymers, prepared, forexample, by reacting trioxane, one of the cyclic ethers described above,and a third monomer, preferably a bifunctional compound of the formula

where Z is a chemical bond, —O—, or —ORO— (R=C₁-C₈-alkylene orC₂-C₈-cycloalkylene).

Preferred monomers of this type are ethylene diglycide, diglycidylether, and diethers made from glycidyl compounds and formaldehyde,dioxane, or trioxane in a molar ratio of 2:1, and also diethers madefrom 2 mol of glycidyl compound and 1 mol of an aliphatic diol havingfrom 2 to 8 carbon atoms, for example the diglycidyl ethers of ethyleneglycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol,1,2-propanediol, or 1,4-cyclohexanediol, to mention merely a fewexamples.

Processes for preparing the POM homo- and copolymers described above areknown to the skilled worker and are described in the literature.

The preferred POM copolymers have melting points of at least 150° C. andmolecular weights (weight-average) M_(w) in the range from 5000 to200,000, preferably from 7000 to 150,000. Particular preference is givento end-group-stabilized POM polymers which have carbon-carbon bonds atthe ends of the chains.

The melt index (MVR 190/2.16) of the POM polymers used is generally from2 to 50 cm³/10 min (ISO 1133).

Conductivity blacks (B) are carbon blacks with a very high degree ofstructure. In these carbon blacks, there is a relationship betweenstructure, surface and particle size. Surface and structure increase asparticle size reduces. A high degree of structure permits achievement ofbetter electrical conductivity. Dibutyl phthalate absorption serves as ameasure of structure. Conductivity blacks which have high activity havedibutyl phthalate absorption above 450 cm³/100 g.

The lubricant mixture (C) is composed of a lubricant with predominantlyexternal lubricant action and of a lubricant with predominantly internallubricant action. The mixing ratio between lubricant with predominantlyinternal lubricant action and lubricant with predominantly externallubricant action may be from 0:100 to 100:0 parts by weight. Lubricantswhich may be used and have predominantly external lubricant action aresolid and/or liquid paraffins, montanic esters, partially saponifiedmontanic esters, stearic acids, polar polyethylene waxes, non-polarpolyethylene waxes, poly-α-olefin oligomers, silicone oils, polyalkyleneglycols, and perfluoroalkyl ethers. Soaps and esters, including thosewhich have been partially saponified, are lubricants with externallubricant action and also lubricants with internal lubricant action.Preference is given to the use of a high-molecular-weight, oxidized andtherefore polar polyethylene wax. This improves tribological performanceand can reduce the severity of fall-off in mechanical properties.Stearyl stearate is preferably used as lubricant with predominantlyinternal lubricant action, and provides gentle conditions forincorporation of the carbon black.

Using the predominantly surface-active oxidized polyethylene wax on itsown in the polyoxymethylene/conductivity black mixture can achievehigher strength, particularly within the weld line, and better wearperformance when comparison is made with a lubricant with predominantlyinternal lubricant action. However, in order to maintain theconductivity of the carbon black the addition of an internal lubricantis necessary.

The particular features of the oxidized polyethylene wax are thefunctional groups having oxygen bonded at the surface. They aregenerated in a controlled manner by oxidative post-treatment. Theoxidative post-treatment of the polyethylene wax improves affinity tothe POM. When comparison is made with other polyethylene waxes, there isa less severe reduction in weld line extensibility, and sliding abrasionis reduced. This is described in patent application EP 0 905 190 A1.Paraffins, solid or liquid, stearic acids, polyethylene waxes, non-polaror polar, poly-α-olefin oligomers, silicone oils, polyalkylene glycols,and perfluoroalkyl ethers are lubricants with predominantly externallubricant action. Soaps and esters, including those which have beenpartially saponified, are lubricants with both external and internallubricant action. Montanic esters and partially saponified montanicesters are lubricants with predominantly external lubricant action.

The preferred oxidized polyethylene wax in component (C) is ahigh-molecular-weight, polar wax, and generally has an acid value offrom 12 to 20 mg KOH/g and a viscosity of from 300 to 5000 mPa*s at 140°C.

Mention may be made of the following lubricants with predominantlyinternal lubricant action within component (C): fatty alcohols,dicarboxylic esters, fatty esters, fatty acids, fatty soaps, fattyamides, wax esters, and stearyl stearates, the last-named beingpreferred. Lubricants are described in Gächter/Müller, “Taschenbuch derKunststoff-Additive” [Plastics Additives Handbook], 3rd Edition, CarlHanser Verlag Munich/Vienna 1994, pp. 478-504, incorporated herein byway of reference.

Preferred impact-modifier components (D) are thermoplasticallyprocessable elastomers, preferably TPEU (thermoplastic polyurethaneelastomers). These materials are multiblock copolymers composed of stiffurethane segments and of flexible long-chain diol segments.

The stiff urethane segments here are obtained from a reaction betweendiisocyanates and what are known as chain extenders. Aromatic,alicyclic, or aliphatic diisocyanates may be used as diisocyanate.Preference is given here to diphenylmethane 4,4′-diisocyanate (MDI),tolylene diisocyanate (TDI), m-xylylene diisocyanate, p-xylylenediisocyanate, naphthylene diisocyanate, methylenebis(cyclohexyl4-isocyanate), isophorone diisocyanate, and hexamethylene1,6-diisocyanate. The chain extenders used comprise short-chainaliphatic, alicyclic, or aromatic diols or diamines with molar massbelow 500 g/mol, preferably below 300 g/mol. Examples of chain extenderswhose use is preferred are ethylene glycol, propylene 1,3-glycol,propylene 1,2-glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,4-cyclohexanediol, ethylenediamine, hexamethylenediamine,xylylenediamine, and 4,4′-diaminodiphenylmethane.

The flexible long-chain diol segments may be selected frompolyetherdiols, polyesterdiols, polyetheresterdiols, andpolycarbonatediols with number-average molar mass of from 500 to 5000g/mol, preferably from 1000 to 3000 g/mol. The polyetherdiols may beobtained by ring-opening polymerization of cyclic C2-C12 ethers, e.g.ethylene oxide, propylene oxide, or tetrahydrofuran. The polyesterdiolsmay be obtained by esterification reactions of dialcohols (examples ofthose preferred here being ethylene glycol, propylene 1,3-glycol,propylene 1,2-glycol, 1,4-butanediol, 1,3-butanediol,2-methylpropanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol,1,6-hexanediol, neopentyl glycol, nonanediol, 1,10-decanediol) anddicarboxylic acids (examples of those preferred here being glutaricacid, adipic acid, pimelic acid, subaric acid, sebacic acid,terephthalic acid, and isophthalic acid), or by correspondingtransesterification reactions. It is also possible to obtainpolyesterdiols of this type by ring-opening polymerization of lactones(examples of those preferred here being caprolactone, propiolactone, andvalerolactone). The polycarbonates may be obtained by the reaction ofdialcohols (examples of those preferred here being ethylene glycol,propylene 1,3-glycol, propylene 1,2-glycol, 1,4-butanediol,1,3-butanediol, 2-methylpropanediol, 1,5-pentanediol,3-methyl-1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, nonanediol,1,10-decanediol) with diphenyl carbonate or phosgene.

Polyester urethanes are preferably used for the molding compositiondescribed. The products lie within the hardness range of from aboutShore A 65 to about Shore D 75. This hardness is also a measure of theproportion of the stiff urethane segments to the flexible long-chaindiol segments. The melt index of the products is measured at varioustemperatures, depending on the melting behavior of the stiff urethanesegments. It is also a measure of the degree of addition (molar mass ofthe entire chains).

The additives and processing aids (E) used may be additives such asformaldehyde scavengers, acid scavengers, antioxidants, UV stabilizers,coupling agents, nucleating agents, and mold-release agents, theproportion of which in component (E) is from 0.005 to 5 parts by weight.Additional substances used for improving electrical conductivity arethose such as antistats, graphite, carbon fibers, and also mixtures ofthese. The proportions by weight of these substances in component (E)are from 0.5 to 20 parts by weight. It is also possible to useadditional lubricants, such as ultrahigh-molecular-weight polyethylene(UHMWPE), aramid fiber, chalk, wollastonite, polytetrafluoroethylene(PTFE), or graft copolymer which is a product of a graft reaction of anolefin polymer with an acrylonitrile-styrene copolymer, or a mixture ofthese, using from 0.5 to 25 parts by weight in component (E).

The molding composition of the invention is suitable for functionalcomponents in which static charge is undesirable or dangerous. It isparticularly suitable for applications which also demand low frictionand low wear. The components produced from the molding composition maybe extruded films or sheets, or else injection moldings. Examples whichmay be mentioned are: functional components, such as level probes forheating-oil tanks, membrane grids in contactless fill-level measurementsystems, strippers on paper-transport-process devices, transport rollersfor transport systems, and chain links, slide rails, and gearwheels.

EXAMPLES

For inventive example 2 and comparative example 1, use was made of acopolymer of trioxane and dioxolane with melt volume index MVR 190/2.16of 8 cm³/10 min. The copolymer was treated with the additives listed inTable 1.

TABLE 1 Example 1 Example 2 Comparative Mixing mixing specification ofspecification parts by weight the invention Parts by weight POMcopolymer 81.2 POM copolymer 85 Conductivity 4 Conductivity black 4black Polyester 10 Polyesterurethane 7 urethane Polyethylene 3 Stearylstearate 2 glycol Non-polar PE 1.8 Polar PE wax 2 wax

The POM copolymer was mixed with the respective additives in a Diosna V100 high-speed fluidizing mixer (Dierks u. Söhne, Osnabrück, Germany)and melted in a ZE 25×33 D twin-screw extruder (Berstorff, Hanover,Germany) using a melt temperature of 200° C., and then pelletized.

These pellets were dried for 8 hours at 120° C. and then injectionmolded to give test specimens for mechanical and tribological testing.The injection molding machine used was model KM 90/210 B (Krauss Maffei,Munich, Germany). The processing conditions were selected in accordancewith the recommendations of ISO 9988-2, standard for POM material.

Tests:

Rheological Properties

MVR 190/2.16 to ISO 1133

Mechanical Properties

Tensile test to ISO 527 Parts 1 and 2

Thermal Properties

Heat resistance HDT/A (1.8 N/mm²) to ISO 75 Parts 1 and 2

Electrical Properties

Volume resistivity to IEC 93

Surface resistivity to IEC 93

Wear Tests

Abrasion was measured on a wear shaft—a rotating shaft onto which arepressed cylindrical test specimens of diameter 12 mm made from thematerial to be tested. The volume of wear is determined as a function oftime. The principle of the test corresponds to the “pin on ring”principle to ISO/DIS 7148-2.

Test Conditions:

Shaft material Steel Shaft diameter 65 mm Roughness depth Rz about 0.8μm Load 3.1 N Sliding velocity 136 m/min Duration of experiments 60 h

Table 2 lists the test results.

TABLE 2 Example 1 Example 2 Comparative mixing Mixing specification ofResults specification the invention Yield stress 34 MPa 50 MPa Heatresistance 67° C. 78° C. Volume resistivity 1100 ohm cm 1030 ohm cmSurface resistivity 500 ohm 580 ohm Volume of wear 25 mm³ 9 mm³

The examples show that in example 2, the mixing specification withoxidized polyethylene wax in combination with stearyl stearate, markedlyhigher yield stress and higher heat resistance are found, whileelectrical properties remain the same as in example 1. Abrasion inexample 2 is likewise markedly lower than in example 1.

The mixing specification of example 2 is a compromise between alubricant with predominantly external lubricant action and a lubricantwith predominantly internal lubricant action.

The action of the individual lubricants alone was investigated inexamples 3 and 4 (Table 3). It can be seen from the results in Table 4that the flowability of the melt has been improved in example 4,providing gentler conditions for incorporation of the conductivityblack. As a result, electrical properties are better in example 4 thanin example 3. However, example 3 has better weld line strength and lowervolume of wear, using a lubricant with predominantly external lubricantaction.

TABLE 3 Example 3 Parts by weight Example 4 Parts by weight POMcopolymer 86 POM copolymer 86 Conductivity 4 Conductivity black 4 blackPolyesterurethane 7 Polyesterurethane 7 Polar PE wax 3 Stearyl stearate3

TABLE 4 Results Example 3 Example 4 MVR 190/2.16 0.34 cm³/10 min 1.15cm³/10 min Yield stress with weld 55 MPa 47 MPa line Volume resistivity8300 ohm cm 4200 ohm cm Surface resistivity 760 ohm 230 ohm Volume ofwear 10.9 mm³ 20.6 mm³

What is claimed is:
 1. A thermoplastic molding composition composed ofcomponent (A) from 14 to 97.5 parts by weight of a polyoxymethylenehomo-or copolymer, component (B) from 1 to 10 parts by weight of aconductivity black, component (C) from 0.5 to 6 parts by weight of alubricant mixture composed of a lubricant with predominantly internallubricant action and of a lubricant with predominantly externallubricant action, component (D) from 1 to 25 parts by weight of animpact-modifier component, and (E) from 0 to 50 parts by weight of otheradditives and processing aids, fillers, reinforcing materials, and/orpolymeric lubricants, the entirety of components (A), (B), and (C)always being 100 parts by weight component and wherein the lubricantwith predominantly internal lubricant action is a stearyl stearate. 2.The molding composition as claimed in claim 1, which is composed of (A)from 60 to 89 parts by weight of component (A), (B) from 3 to 5 parts byweight of component (B), (C) from 3 to 5 parts by weight of component(C), (D) from 5 to 10 parts by weight of an impact-modifier component(D).
 3. The molding composition as claimed in claim 2, wherein saidcomponent (D) is a polyesterurethane.
 4. The molding composition asclaimed in claim 3, wherein said component (E) is one or more materialsselected from the group consisting of chalk, talc, wollastonite, mica,zinc oxide, silicon dioxide, the reinforcing materials glass fibers,carbon fibers, organic high-modulus fibers, polymeric lubricantspolytetrafluoroethylene in pulverulent form, ethylene in fiber form,UHMW polyethylene and graft polymers obtained from a graft reaction ofpolyethylene with acrylonitrile-styrene copolymer (SAN).
 5. The moldingcomposition as claimed in claim 1, wherein said lubricant withpredominantly external lubricant action is selected from the groupconsisting of solid paraffin, liquid paraffin, montanic esters,partially saponified montanic esters, stearic acids, polar polyethylenewaxes, non-polar polyethylene waxes, poly-α-olefin oligomers, siliconeoils, polyalkylene glycols and perfluoroalkyl ethers.
 6. The moldingcomposition as claimed in claim 1, wherein said lubricant withpredominantly external lubricant action is a high-molecular-weight,polar polyethylene wax and has an acid value of from 12 to 20 mg KOH/gand a viscosity of from 3000 to 5000 mPa*s at a temperature of 140° C.7. The molding composition as claimed in claim 1, wherein said component(D) is a thermoplastic polyurethane elastomer.
 8. The moldingcomposition as claimed in claim 1, wherein said component (D) is apolyesterurethane.
 9. The molding composition as claimed in claim 1,wherein said component (E) is one or more materials selected from thegroup consisting of chalk, talc, wollastonite, mica, zinc oxide, silicondioxide, the reinforcing materials glass fibers, carbon fibers, organichigh-modulus fibers, polymeric lubricants polytetrafluoroethylene inpulverulent form, ethylene in fiber form, UHMW polyethylene and graftpolymers obtained from a graft reaction of polyethylene withacrylonitrile-styrene copolymer (SAN).
 10. A film comprising the moldingcomposition as claimed in claim
 1. 11. A function component withrequirements for good electrical conductivity and good wear performancewhich comprise the molding composition as claimed in claim 1 and whereinthe function component is selected from the group consisting of levelprobes for heating oil tanks, membrane grids in contactless fill-levelmeasurement systems, chain links, slide rails and gearwheels.
 12. Amolding produced from a thermoplastic molding composition as claimed inclaim 1.